The present invention relates to a process for the deposition of boron carbide semiconductor material, and also to semiconductor devices formed by deposition of a boron carbide film. The invention is more particularly directed to a technique for creating a layer of boron carbide with a boron-to-carbon ratio of about 5. The invention is also particularly directed to semiconductor devices produced by this technique.
Boron and boron carbide devices have been sought since 1959 [1] but only recently has the fabrication of these devices been realized [2, 3]. Such devices would have applicability in a wide number of harsh conditions. For example, they should be resistant to corrosive, high temperature, and mechanically abrasive environments. Because of the large neutron capture cross-section, these materials could be used in devices in radioactive environments as well [4].
Techniques are known for forming boron-rich carbides. These techniques may employ alkanes and heavy boron cage molecules to deposit boron carbide thin films. Plasma-enhanced chemical vapor deposition (PECVD) can be employed to fabricate boron carbide films without resort to high temperatures or high pressures. These technique typically employ a halide of boron, e.g., BCl3, BBr3 or Bl3. Most recently boranes, such as nido-decaborane and nido-pentaborane have gained interest, because these compounds are safe and stable, yet produce a vapor pressure of several Torr at room temperature. However, until very recently, only low-resistivity boron carbide materials could be produced, i.e., materials with resistivities on the order of about ten ohm-cm at room temperature. Boron carbide material of this type has an extremely low band gap, and is not suited as a semiconductor material.
At the same time, boron carbide has become an attractive material because of its inherent hardness and durability. Boron carbide, like other boron-containing materials, has been considered for high temperature electronic devices because it retains its useful characteristics at elevated temperatures. For example, boron carbide is known to have a melting temperature of 2350xc2x0 C., a strength of 50 ksi, a hardness of 2800 kg/mm2, and a thermal conductivity of 0.22 cal/cm/sec/xc2x0 C./cm. Diamond and silicon carbide have been investigated because of their good thermal and mechanical characteristics, and because of their wide band gaps. However, these materials have not yet proven cost effective.
Recent successes in construction of boron carbide/n-Si [1,1,1] heterojunction diodes [5,6] have demonstrated that boron carbide/Si [1,1,1] heterojunction diodes can be fabricated from closo-1,2-dicarbadodecaborane (C2B10H12; orthocarborane) by using synchrotron radiation induced chemical vapor deposition (SR-CVD) [5, 6], and plasma enhanced chemical vapor deposition (PECVD) [2, 3, 5-7]. Pure boron films also had been deposited on silicon from nido-decaborane (B10H14; decaborane) by using SR-CVD [8, 9]. Boron carbide/n-Si [1,1,1] heterojunction devices fabricated by depositing boron carbide from nido-pentaborane and alkanes using PECVD is shown and described in U.S. Pat. No. 5,468,978, hereby incorporated by reference in its entirety.
It is an object of the present invention to provide boron/boron carbide and doped boron carbide semiconductor devices and techniques to fabricate same.
It is another object of the present invention to provide a semiconductor device suited for use in high temperature, corrosive, mechanically abrasive, or radioactive environments.
Another object of the present invention is to provide boron carbide semiconductor devices and fabrication techniques which do not require a silicon interface.
Yet another object of the present invention is to provide boron carbide semiconductor devices and fabrication techniques to fabricate same which do not depend on epitaxial growth or crystallite size.
Still another object of the present invention is to provide doped boron carbide semiconductor devices and fabrication techniques for making same. In one embodiment, the doped boron carbide appears as n-type relative to an n-type silicon substrate.
The objects of the present invention are provided by the boron/boron carbide heterojunction devices and the doped boron carbide/silicon semiconductor devices and fabrication techniques as disclosed herein.
The fabrication of several working boron/boron carbide semiconductor devices is described herein. In an effort to fabricate a more sophisticated device, a transistor was made in a PECVD system. A diode was made directly on an aluminum substrate to demonstrate that a silicon interface is not essential for fabrication of a boron carbide device. The use of plasma enhanced chemical vapor deposition (PECVD) provides a means for fabricating boron and boron carbide thin films successfully in a high resistivity form [2, 3]. The present invention addresses some of the issues associated with making devices of increasing complexity from boron carbide.
The aluminum substrates were polycrystalline, and the silicon substrates were [1,1,1], n-type. Both were chemically etched and cleaned prior to insertion in vacuo and set on the lower electrode. The substrates were further cleaned by Ar+ bombardment at 300 mTorr, 40 W and annealed at 400xc2x0 C. in the vacuum system. Deposition was carried out in a custom designed parallel plate 13.56 MHZ radio-frequency plasma enhanced chemical vapor deposition (PECVD) reactor described previously [3, 7]. A suitable plasma chamber in which this technique can be carried out is shown and described in U.S. Pat. No. 4,957,773, hereby incorporated by reference in its entirety.
The above, and many other objects and advantages of the present invention will become apparent from the ensuing detailed description of the invention, to be read in conjunction with the accompanying drawings.